The present invention relates to a communication control method in a process control system or the like and, more particularly, to a communication control method in a control system that includes a controller for communicating in a token passing system.
When a token passing system is used in a process control system, it may be structured such that an I/O station takes a token indirectly, i.e., takes instructions from a predetermined controller having control of the token, the I/O station performing an input/output (I/O) operation in response to the instruction. Since the I/O station does not take (control of) the token, the controller cannot verify that the instruction was accurately received and cannot verify the output operation performed by the I/O station in response to the instruction.
However, if the I/O station is to take the token, the I/O station takes the token each time an input/output operation is performed. In process control systems, it generally takes a long time for the I/O station to take the token and perform the software processing, resulting in a slow communication speed. In addition, in order to communicate by taking control of the token in such a manner, the I/O station requires both hardware and software implementations for communication, resulting in high cost.
According to the preferred embodiment of the present invention, a controller which has obtained the token, sends a write instruction with a response request to a station and holds index data sent back from the station which has received the above instruction and indicates a store position of status information corresponding to the above instruction. When the controller subsequently obtains the token, the controller retrieves and confirms corresponding status information on the station using the above index data, thereby permitting the controller to verify the I/O operation in response to the instruction without requiring the I/O station to obtain control of the token.